1. Field of the Invention
The present invention relates in general to a solid state image sensor, and more particularly to a solid state image sensor having a vertical overflow drain structure and using a plurality of photo diodes for photoelectric conversion, and using a plurality of charge coupled devices (hereinbelow, referred to simply as "the CCD") for signal charge transmission, and suitable for used in the case of requiring lower smear and lower blooming.
2. Description of the Prior Art
A description of a problem introduced by a conventional solid state image sensor will be given as referring to an interline type CCD solid state image sensor formed on a N-type silicon wafer, for an example.
With reference to FIG. 8, there is shown in a cross sectional view a structure of a pixel formed on a silicon substrate of a two-dimensional solid state image sensor. The pixel structure includes a first well layer 2 of a predetermined thickness deposited on a surface of a N-type silicon substrate 1 to cover all of the pixel region. The pixel structure further includes a second well layer 3 which has a thickness substantially less than that of the first well layer 2 and covers a CCD channel region 4.
The incident light reaching the surface of the CCD through a given optical system is received by a photo diode 6 through an opening of a light blanket layer 11. Here, the second well layer 3 serves to block the electrons generated by the incident light inside the first well layer 2 and to restrict the smear caused by leakage of the electrons into the CCD channel 4.
The N-type silicon substrate 1 is biased by a positive electric potential so that when there is generated in the photo diode an excessive charge, exceeding the capacity of the photo diode 6, the electrons are emitted from the photo diode 6 to the silicon substrate 1.
A problem of the aforementioned solid state image sensor is resided in that only the smallest part of the incident light received by the surface of the CCD can be photoelectrically converted.
One approach to enlargement of the part of the incident light to be photoelectrically converted is to use a lens provided on a surface of the CCD corresponding to each of the pixels to focus the incident light on a corresponding photo diode 6.
FIG. 9 shows a sectional view of pixels of the solid state image sensor to comparatively show a light focusing effect of a lens provided on the CCD surface. In this drawing, the left-side pixel is provided with a micro lens 19, however, the right-side pixel has no lens.
As represented by the light paths 20 of the incident lights of FIG. 9, the light are focused on the photo diode 6 by the micro lens 19. In this regard, it is noted that the using efficiency of the incident light is remarkably improved by the micro lens 19. It is known to those skilled in the art that the use of the lens 19 achieves an improvement of the sensitivity of two or three times as much as the case of no use .of the lens.
However, the use of the micro lens 19 also causes a problem that it increases the smear due to the following reason.
Conventionally, all of the incident light beams can not be vertically received by the surface of the CCD, but only a light beam passing through the center of the optical system is vertically received by the surface of the CCD since there is an angular distribution in the incident angle of the light depending upon the types of optical systems.
That is, when enlarging the iris used for adjusting the quantity of light, the incident angle distribution is also enlarged. However, when making this iris narrow, the incident angle distribution is also narrowed.
In addition, the incident angle distribution also shows a difference between the center of and the periphery of the light receiving section of the CCD. Otherwise stated, the incident light at the periphery of the light receiving section of the CCD is received by the CCD surface at an incident angle diverging at the maximum from the perpendicular of the CCD.
FIG. 10 shows incident light paths when the light beams are received by the CCD through light paths diverging from the perpendicular of the CCD.
As shown in this drawing, there is no change in the incident angle of the light beam when there is no lens provided on the CCD, however, when the lens 19 is provided on the CCD, the light beam is refracted by the lens 19 to increase the divergence of the incident light paths from the perpendicular of the CCD, thereby causing the light paths to get .near the CCD channel 4.
Since the second well layer 3 serves as a barrier for the CCD channel 4, the fact that the light paths get near the CCD channel 4 causes no smear. However, when an intensive light of which the quantity of light is 1,000 times as much as the saturated quantity of light is received by the CCD through the same light paths described above, there is a variation of the electric potential due to the electric charge generated by the intensive incident light and this causes a great number of electrons to be rushed into the CCD channel 4.
The ratio of the light component reflected by the surface of the silicon substrate 1 is increased in proportion to the divergence of the incident light from the perpendicular of the CCD, so that the light component repeatedly reflected by both the silicon surface and the light blanket layer 11 prior to its incidence to the photo diode 6 is increased.
It is possible to prevent the smear, caused by the divergence of the incident light from the perpendicular of the CCD, by making narrow the gap between the silicon surface and the light blanket layer 11.
However, the known technology can not prevent the smear, caused by the electrons rushed into the CCD channel 4 when the intensive light of which the quantity of light is 1,000 times as much as the saturated quantity of light is received by the CCD.
If briefly described, the known CCD solid state image sensor has a serious problem that the smear according to variation of the incident angle distribution is such remarkable that it can not be prevented by the use of lens provided on the surface of the CCD.